Apache Performance Tuning

Apache 2.x is a general-purpose webserver, designed to
provide a balance of flexibility, portability, and performance.
Although it has not been designed specifically to set benchmark
records, Apache 2.x is capable of high performance in many
real-world situations.

Compared to Apache 1.3, release 2.x contains many additional
optimizations to increase throughput and scalability. Most of
these improvements are enabled by default. However, there are
compile-time and run-time configuration choices that can
significantly affect performance. This document describes the
options that a server administrator can configure to tune the
performance of an Apache 2.x installation. Some of these
configuration options enable the httpd to better take advantage
of the capabilities of the hardware and OS, while others allow
the administrator to trade functionality for speed.

The single biggest hardware issue affecting webserver
performance is RAM. A webserver should never ever have to swap,
as swapping increases the latency of each request beyond a point
that users consider "fast enough". This causes users to hit
stop and reload, further increasing the load. You can, and
should, control the MaxRequestWorkers setting so that your server
does not spawn so many children it starts swapping. This procedure
for doing this is simple: determine the size of your average Apache
process, by looking at your process list via a tool such as
top, and divide this into your total available memory,
leaving some room for other processes.

Beyond that the rest is mundane: get a fast enough CPU, a
fast enough network card, and fast enough disks, where "fast
enough" is something that needs to be determined by
experimentation.

Operating system choice is largely a matter of local
concerns. But some guidelines that have proven generally
useful are:

Run the latest stable release and patchlevel of the
operating system that you choose. Many OS suppliers have
introduced significant performance improvements to their
TCP stacks and thread libraries in recent years.

If your OS supports a sendfile(2) system
call, make sure you install the release and/or patches
needed to enable it. (With Linux, for example, this means
using Linux 2.4 or later. For early releases of Solaris 8,
you may need to apply a patch.) On systems where it is
available, sendfile enables Apache 2 to deliver
static content faster and with lower CPU utilization.

Prior to Apache 1.3, HostnameLookups defaulted to On.
This adds latency to every request because it requires a
DNS lookup to complete before the request is finished. In
Apache 1.3 this setting defaults to Off. If you need
to have addresses in your log files resolved to hostnames, use the
logresolve
program that comes with Apache, or one of the numerous log
reporting packages which are available.

It is recommended that you do this sort of postprocessing of
your log files on some machine other than the production web
server machine, in order that this activity not adversely affect
server performance.

If you use any Allow from domain or Deny from domain
directives (i.e., using a hostname, or a domain name, rather than
an IP address) then you will pay for
two DNS lookups (a reverse, followed by a forward lookup
to make sure that the reverse is not being spoofed). For best
performance, therefore, use IP addresses, rather than names, when
using these directives, if possible.

Note that it's possible to scope the directives, such as
within a <Location /server-status> section.
In this case the DNS lookups are only performed on requests
matching the criteria. Here's an example which disables lookups
except for .html and .cgi files:

Wherever in your URL-space you do not have an Options
FollowSymLinks, or you do have an Options
SymLinksIfOwnerMatch Apache will have to issue extra
system calls to check up on symlinks. One extra call per
filename component. For example, if you had:

and a request is made for the URI /index.html.
Then Apache will perform lstat(2) on
/www, /www/htdocs, and
/www/htdocs/index.html. The results of these
lstats are never cached, so they will occur on
every single request. If you really desire the symlinks
security checking you can do something like this:

This at least avoids the extra checks for the
DocumentRoot path.
Note that you'll need to add similar sections if you
have any Alias or
RewriteRule paths
outside of your document root. For highest performance,
and no symlink protection, set FollowSymLinks
everywhere, and never set SymLinksIfOwnerMatch.

Wherever in your URL-space you allow overrides (typically
.htaccess files) Apache will attempt to open
.htaccess for each filename component. For
example,

DocumentRoot /www/htdocs
<Directory />
AllowOverride all
</Directory>

and a request is made for the URI /index.html.
Then Apache will attempt to open /.htaccess,
/www/.htaccess, and
/www/htdocs/.htaccess. The solutions are similar
to the previous case of Options FollowSymLinks.
For highest performance use AllowOverride None
everywhere in your filesystem.

If at all possible, avoid content-negotiation if you're
really interested in every last ounce of performance. In
practice the benefits of negotiation outweigh the performance
penalties. There's one case where you can speed up the server.
Instead of using a wildcard such as:

DirectoryIndex index

Use a complete list of options:

DirectoryIndex index.cgi index.pl index.shtml index.html

where you list the most common choice first.

Also note that explicitly creating a type-map
file provides better performance than using
MultiViews, as the necessary information can be
determined by reading this single file, rather than having to
scan the directory for files.

If your site needs content negotiation consider using
type-map files, rather than the Options
MultiViews directive to accomplish the negotiation. See the
Content Negotiation
documentation for a full discussion of the methods of negotiation,
and instructions for creating type-map files.

Memory-mapping

In situations where Apache 2.x needs to look at the contents
of a file being delivered--for example, when doing server-side-include
processing--it normally memory-maps the file if the OS supports
some form of mmap(2).

On some platforms, this memory-mapping improves performance.
However, there are cases where memory-mapping can hurt the performance
or even the stability of the httpd:

On some operating systems, mmap does not scale
as well as read(2) when the number of CPUs increases.
On multiprocessor Solaris servers, for example, Apache 2.x sometimes
delivers server-parsed files faster when mmap is disabled.

If you memory-map a file located on an NFS-mounted filesystem
and a process on another NFS client machine deletes or truncates
the file, your process may get a bus error the next time it tries
to access the mapped file content.

For installations where either of these factors applies, you
should use EnableMMAP off to disable the memory-mapping
of delivered files. (Note: This directive can be overridden on
a per-directory basis.)

Sendfile

In situations where Apache 2.x can ignore the contents of the file
to be delivered -- for example, when serving static file content --
it normally uses the kernel sendfile support the file if the OS
supports the sendfile(2) operation.

On most platforms, using sendfile improves performance by eliminating
separate read and send mechanics. However, there are cases where using
sendfile can harm the stability of the httpd:

Some platforms may have broken sendfile support that the build
system did not detect, especially if the binaries were built on
another box and moved to such a machine with broken sendfile support.

With an NFS-mounted filesystem, the kernel may be unable
to reliably serve the network file through its own cache.

For installations where either of these factors applies, you
should use EnableSendfile off to disable sendfile
delivery of file contents. (Note: This directive can be overridden
on a per-directory basis.)

Prior to Apache 1.3 the MinSpareServers, MaxSpareServers, and StartServers settings all had drastic effects on
benchmark results. In particular, Apache required a "ramp-up"
period in order to reach a number of children sufficient to serve
the load being applied. After the initial spawning of
StartServers children,
only one child per second would be created to satisfy the
MinSpareServers
setting. So a server being accessed by 100 simultaneous
clients, using the default StartServers of 5 would take on
the order 95 seconds to spawn enough children to handle
the load. This works fine in practice on real-life servers,
because they aren't restarted frequently. But does really
poorly on benchmarks which might only run for ten minutes.

The one-per-second rule was implemented in an effort to
avoid swamping the machine with the startup of new children. If
the machine is busy spawning children it can't service
requests. But it has such a drastic effect on the perceived
performance of Apache that it had to be replaced. As of Apache
1.3, the code will relax the one-per-second rule. It will spawn
one, wait a second, then spawn two, wait a second, then spawn
four, and it will continue exponentially until it is spawning
32 children per second. It will stop whenever it satisfies the
MinSpareServers
setting.

This appears to be responsive enough that it's almost
unnecessary to twiddle the MinSpareServers, MaxSpareServers and StartServers knobs. When more than 4 children are
spawned per second, a message will be emitted to the
ErrorLog. If you
see a lot of these errors then consider tuning these settings.
Use the mod_status output as a guide.

Related to process creation is process death induced by the
MaxConnectionsPerChild
setting. By default this is 0,
which means that there is no limit to the number of connections
handled per child. If your configuration currently has this set
to some very low number, such as 30, you may want to bump this
up significantly. If you are running SunOS or an old version of
Solaris, limit this to 10000 or so because of memory leaks.

When keep-alives are in use, children will be kept busy
doing nothing waiting for more requests on the already open
connection. The default KeepAliveTimeout of 5
seconds attempts to minimize this effect. The tradeoff here is
between network bandwidth and server resources. In no event
should you raise this above about 60 seconds, as
most of the benefits are lost.

Choosing an MPM

Apache 2.x supports pluggable concurrency models, called
Multi-Processing Modules (MPMs).
When building Apache, you must choose an MPM to use. There
are platform-specific MPMs for some platforms:
mpm_netware,
mpmt_os2, and mpm_winnt. For
general Unix-type systems, there are several MPMs from which
to choose. The choice of MPM can affect the speed and scalability
of the httpd:

The worker MPM uses multiple child
processes with many threads each. Each thread handles
one connection at a time. Worker generally is a good
choice for high-traffic servers because it has a smaller
memory footprint than the prefork MPM.

The event MPM is threaded like the
Worker MPM, but is designed to allow more requests to be
served simultaneously by passing off some processing work
to supporting threads, freeing up the main threads to work
on new requests.

The prefork MPM uses multiple child
processes with one thread each. Each process handles
one connection at a time. On many systems, prefork is
comparable in speed to worker, but it uses more memory.
Prefork's threadless design has advantages over worker
in some situations: it can be used with non-thread-safe
third-party modules, and it is easier to debug on platforms
with poor thread debugging support.

For more information on these and other MPMs, please
see the MPM documentation.

Since memory usage is such an important consideration in
performance, you should attempt to eliminate modules that you are
not actually using. If you have built the modules as DSOs, eliminating modules is a simple
matter of commenting out the associated LoadModule directive for that module.
This allows you to experiment with removing modules, and seeing
if your site still functions in their absence.

If, on the other hand, you have modules statically linked
into your Apache binary, you will need to recompile Apache in
order to remove unwanted modules.

An associated question that arises here is, of course, what
modules you need, and which ones you don't. The answer here
will, of course, vary from one web site to another. However, the
minimal list of modules which you can get by with tends
to include mod_mime, mod_dir,
and mod_log_config. mod_log_config is,
of course, optional, as you can run a web site without log
files. This is, however, not recommended.

Atomic Operations

Some modules, such as mod_cache and
recent development builds of the worker MPM, use APR's
atomic API. This API provides atomic operations that can
be used for lightweight thread synchronization.

By default, APR implements these operations using the
most efficient mechanism available on each target
OS/CPU platform. Many modern CPUs, for example, have
an instruction that does an atomic compare-and-swap (CAS)
operation in hardware. On some platforms, however, APR
defaults to a slower, mutex-based implementation of the
atomic API in order to ensure compatibility with older
CPU models that lack such instructions. If you are
building Apache for one of these platforms, and you plan
to run only on newer CPUs, you can select a faster atomic
implementation at build time by configuring Apache with
the --enable-nonportable-atomics option:

The --enable-nonportable-atomics option is
relevant for the following platforms:

Solaris on SPARC
By default, APR uses mutex-based atomics on Solaris/SPARC.
If you configure with --enable-nonportable-atomics,
however, APR generates code that uses a SPARC v8plus opcode for
fast hardware compare-and-swap. If you configure Apache with
this option, the atomic operations will be more efficient
(allowing for lower CPU utilization and higher concurrency),
but the resulting executable will run only on UltraSPARC
chips.

Linux on x86
By default, APR uses mutex-based atomics on Linux. If you
configure with --enable-nonportable-atomics,
however, APR generates code that uses a 486 opcode for fast
hardware compare-and-swap. This will result in more efficient
atomic operations, but the resulting executable will run only
on 486 and later chips (and not on 386).

mod_status and ExtendedStatus On

If you include mod_status and you also set
ExtendedStatus On when building and running
Apache, then on every request Apache will perform two calls to
gettimeofday(2) (or times(2)
depending on your operating system), and (pre-1.3) several
extra calls to time(2). This is all done so that
the status report contains timing indications. For highest
performance, set ExtendedStatus off (which is the
default).

accept Serialization - multiple sockets

Warning:

This section has not been fully updated
to take into account changes made in the 2.x version of the
Apache HTTP Server. Some of the information may still be
relevant, but please use it with care.

This discusses a shortcoming in the Unix socket API. Suppose
your web server uses multiple Listen statements to listen on either multiple
ports or multiple addresses. In order to test each socket
to see if a connection is ready Apache uses
select(2). select(2) indicates that a
socket has zero or at least one connection
waiting on it. Apache's model includes multiple children, and
all the idle ones test for new connections at the same time. A
naive implementation looks something like this (these examples
do not match the code, they're contrived for pedagogical
purposes):

But this naive implementation has a serious starvation problem.
Recall that multiple children execute this loop at the same
time, and so multiple children will block at
select when they are in between requests. All
those blocked children will awaken and return from
select when a single request appears on any socket
(the number of children which awaken varies depending on the
operating system and timing issues). They will all then fall
down into the loop and try to accept the
connection. But only one will succeed (assuming there's still
only one connection ready), the rest will be blocked
in accept. This effectively locks those children
into serving requests from that one socket and no other
sockets, and they'll be stuck there until enough new requests
appear on that socket to wake them all up. This starvation
problem was first documented in PR#467. There
are at least two solutions.

One solution is to make the sockets non-blocking. In this
case the accept won't block the children, and they
will be allowed to continue immediately. But this wastes CPU
time. Suppose you have ten idle children in
select, and one connection arrives. Then nine of
those children will wake up, try to accept the
connection, fail, and loop back into select,
accomplishing nothing. Meanwhile none of those children are
servicing requests that occurred on other sockets until they
get back up to the select again. Overall this
solution does not seem very fruitful unless you have as many
idle CPUs (in a multiprocessor box) as you have idle children,
not a very likely situation.

Another solution, the one used by Apache, is to serialize
entry into the inner loop. The loop looks like this
(differences highlighted):

The functionsaccept_mutex_on and accept_mutex_off
implement a mutual exclusion semaphore. Only one child can have
the mutex at any time. There are several choices for
implementing these mutexes. The choice is defined in
src/conf.h (pre-1.3) or
src/include/ap_config.h (1.3 or later). Some
architectures do not have any locking choice made, on these
architectures it is unsafe to use multiple
Listen
directives.

The Mutex directive can
be used to change the mutex implementation of the
mpm-accept mutex at run-time. Special considerations
for different mutex implementations are documented with that
directive.

Another solution that has been considered but never
implemented is to partially serialize the loop -- that is, let
in a certain number of processes. This would only be of
interest on multiprocessor boxes where it's possible multiple
children could run simultaneously, and the serialization
actually doesn't take advantage of the full bandwidth. This is
a possible area of future investigation, but priority remains
low because highly parallel web servers are not the norm.

Ideally you should run servers without multiple
Listen
statements if you want the highest performance.
But read on.

accept Serialization - single socket

The above is fine and dandy for multiple socket servers, but
what about single socket servers? In theory they shouldn't
experience any of these same problems because all children can
just block in accept(2) until a connection
arrives, and no starvation results. In practice this hides
almost the same "spinning" behaviour discussed above in the
non-blocking solution. The way that most TCP stacks are
implemented, the kernel actually wakes up all processes blocked
in accept when a single connection arrives. One of
those processes gets the connection and returns to user-space,
the rest spin in the kernel and go back to sleep when they
discover there's no connection for them. This spinning is
hidden from the user-land code, but it's there nonetheless.
This can result in the same load-spiking wasteful behaviour
that a non-blocking solution to the multiple sockets case
can.

For this reason we have found that many architectures behave
more "nicely" if we serialize even the single socket case. So
this is actually the default in almost all cases. Crude
experiments under Linux (2.0.30 on a dual Pentium pro 166
w/128Mb RAM) have shown that the serialization of the single
socket case causes less than a 3% decrease in requests per
second over unserialized single-socket. But unserialized
single-socket showed an extra 100ms latency on each request.
This latency is probably a wash on long haul lines, and only an
issue on LANs. If you want to override the single socket
serialization you can define
SINGLE_LISTEN_UNSERIALIZED_ACCEPT and then
single-socket servers will not serialize at all.

Lingering Close

As discussed in
draft-ietf-http-connection-00.txt section 8, in order for
an HTTP server to reliably implement the
protocol it needs to shutdown each direction of the
communication independently (recall that a TCP connection is
bi-directional, each half is independent of the other).

When this feature was added to Apache it caused a flurry of
problems on various versions of Unix because of a
shortsightedness. The TCP specification does not state that the
FIN_WAIT_2 state has a timeout, but it doesn't prohibit it.
On systems without the timeout, Apache 1.2 induces many sockets
stuck forever in the FIN_WAIT_2 state. In many cases this
can be avoided by simply upgrading to the latest TCP/IP patches
supplied by the vendor. In cases where the vendor has never
released patches (i.e., SunOS4 -- although folks with
a source license can patch it themselves) we have decided to
disable this feature.

There are two ways of accomplishing this. One is the socket
option SO_LINGER. But as fate would have it, this
has never been implemented properly in most TCP/IP stacks. Even
on those stacks with a proper implementation (i.e.,
Linux 2.0.31) this method proves to be more expensive (cputime)
than the next solution.

For the most part, Apache implements this in a function
called lingering_close (in
http_main.c). The function looks roughly like
this:

This naturally adds some expense at the end of a connection,
but it is required for a reliable implementation. As HTTP/1.1
becomes more prevalent, and all connections are persistent,
this expense will be amortized over more requests. If you want
to play with fire and disable this feature you can define
NO_LINGCLOSE, but this is not recommended at all.
In particular, as HTTP/1.1 pipelined persistent connections
come into use lingering_close is an absolute
necessity (and
pipelined connections are faster, so you want to support
them).

Scoreboard File

Apache's parent and children communicate with each other
through something called the scoreboard. Ideally this should be
implemented in shared memory. For those operating systems that
we either have access to, or have been given detailed ports
for, it typically is implemented using shared memory. The rest
default to using an on-disk file. The on-disk file is not only
slow, but it is unreliable (and less featured). Peruse the
src/main/conf.h file for your architecture and
look for either USE_MMAP_SCOREBOARD or
USE_SHMGET_SCOREBOARD. Defining one of those two
(as well as their companions HAVE_MMAP and
HAVE_SHMGET respectively) enables the supplied
shared memory code. If your system has another type of shared
memory, edit the file src/main/http_main.c and add
the hooks necessary to use it in Apache. (Send us back a patch
too please.)

Historical note: The Linux port of Apache didn't start to
use shared memory until version 1.2 of Apache. This oversight
resulted in really poor and unreliable behaviour of earlier
versions of Apache on Linux.

DYNAMIC_MODULE_LIMIT

If you have no intention of using dynamically loaded modules
(you probably don't if you're reading this and tuning your
server for every last ounce of performance) then you should add
-DDYNAMIC_MODULE_LIMIT=0 when building your
server. This will save RAM that's allocated only for supporting
dynamically loaded modules.

Note the lack of accept(2) serialization. On this
particular platform, the worker MPM uses an unserialized accept by
default unless it is listening on multiple ports.

/65: lwp_park(0x00000000, 0) = 0
/67: lwp_unpark(65, 1) = 0

Upon accepting the connection, the listener thread wakes up
a worker thread to do the request processing. In this trace,
the worker thread that handles the request is mapped to LWP #65.

/65: getsockname(9, 0x00200BA4, 0x00200BC4, 1) = 0

In order to implement virtual hosts, Apache needs to know
the local socket address used to accept the connection. It
is possible to eliminate this call in many situations (such
as when there are no virtual hosts, or when
Listen directives
are used which do not have wildcard addresses). But
no effort has yet been made to do these optimizations.

/65: brk(0x002170E8) = 0
/65: brk(0x002190E8) = 0

The brk(2) calls allocate memory from the heap.
It is rare to see these in a system call trace, because the httpd
uses custom memory allocators (apr_pool and
apr_bucket_alloc) for most request processing.
In this trace, the httpd has just been started, so it must
call malloc(3) to get the blocks of raw memory
with which to create the custom memory allocators.

Next, the worker thread puts the connection to the client (file
descriptor 9) in non-blocking mode. The setsockopt(2)
and getsockopt(2) calls are a side-effect of how
Solaris' libc handles fcntl(2) on sockets.

This httpd has been configured with Options FollowSymLinks
and AllowOverride None. Thus it doesn't need to
lstat(2) each directory in the path leading up to the
requested file, nor check for .htaccess files.
It simply calls stat(2) to verify that the file:
1) exists, and 2) is a regular file, not a directory.

/65: sendfilev(0, 9, 0x00200F90, 2, 0xFAF7B53C) = 10269

In this example, the httpd is able to send the HTTP response
header and the requested file with a single sendfilev(2)
system call. Sendfile semantics vary among operating systems. On some other
systems, it is necessary to do a write(2) or
writev(2) call to send the headers before calling
sendfile(2).

/65: write(4, " 1 2 7 . 0 . 0 . 1 - ".., 78) = 78

This write(2) call records the request in the
access log. Note that one thing missing from this trace is a
time(2) call. Unlike Apache 1.3, Apache 2.x uses
gettimeofday(3) to look up the time. On some operating
systems, like Linux or Solaris, gettimeofday has an
optimized implementation that doesn't require as much overhead
as a typical system call.

Finally the worker thread closes the file that it has just delivered
and blocks until the listener assigns it another connection.

/67: accept(3, 0x001FEB74, 0x001FEB94, 1) (sleeping...)

Meanwhile, the listener thread is able to accept another connection
as soon as it has dispatched this connection to a worker thread (subject
to some flow-control logic in the worker MPM that throttles the listener
if all the available workers are busy). Though it isn't apparent from
this trace, the next accept(2) can (and usually does, under
high load conditions) occur in parallel with the worker thread's handling
of the just-accepted connection.

Notice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.